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Open Clusters with Hipparcos I. Mean Astrometric Parameters N Open clusters with Hipparcos I. Mean astrometric parameters N. Robichon, Frédéric Arenou, J.-C Mermilliod, C. Turon To cite this version: N. Robichon, Frédéric Arenou, J.-C Mermilliod, C. Turon. Open clusters with Hipparcos I. Mean astrometric parameters. Astronomy and Astrophysics - A&A, EDP Sciences, 1999, 345 (2), pp.471- 484. hal-02053849 HAL Id: hal-02053849 https://hal.archives-ouvertes.fr/hal-02053849 Submitted on 1 Mar 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Astron. Astrophys. 345, 471–484 (1999) ASTRONOMY AND ASTROPHYSICS Open clusters with Hipparcos? I. Mean astrometric parameters N. Robichon1,2, F. Arenou2, J.-C. Mermilliod3, and C. Turon2 1 Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands 2 Observatoire de Paris, section de Meudon, DASGAL/CNRS URA 335, F-92195 Meudon CEDEX, France (Noel.Robichon, Frederic.Arenou, [email protected]) 3 Institut d’Astronomie de Lausanne, CH-1290 Chavannes des Bois, Switzerland ([email protected]) Received 9 December 1998 / Accepted 19 February 1999 Abstract. New memberships, mean parallaxes and proper mo- absolute position of the main sequences of several open clusters tions of all 9 open clusters closer than 300 pc (except the Hyades) independently of any preliminary knowledge of the chemical and 9 rich clusters between 300 and 500 pc have been computed composition. According to the present data on chemical compo- using Hipparcos data. Precisions, ranging from 0.2 to 0.5 mas sition, no large discrepancies are found between the Hipparcos for parallaxes and 0.1 to 0.5 mas/yr for proper motions, are of distance moduli of most of the cluster and the positions of their great interest for calibrating photometric parallaxes as well as sequences in the HR diagram (Mermilliod et al. 1997a, Robi- for kinematical studies. Careful investigations of possible biases chon et al. 1997), with the noticeable exception of the Pleiades. have been performed and no evidence of significant systematic Because the Main-Sequence Fitting (MSF) method is still the errors on the mean cluster parallaxes has been found. The dis- basic tool in determining the distances of open clusters, the tances and proper motions of 32 more distant clusters, which understanding of the Pleiades anomaly appears to be the first may be used statistically, are also indicated. priority. Pinsonneault et al. (1998) (herafter PSSKH) have tackled Key words: stars: distances – Galaxy: open clusters and asso- the problem with a grid of models adapted to the mass range ciations: general of solar-type stars which are unevolved in nearby clusters, and chemical composition of these clusters. Their method deter- mines the distance modulus and metallicity simultaneously from 1. Introduction (MV , (B V )0) and (MV , (V I)0), using the fact that (V I) is much less− sensitive to the metallicity− than (B V ). Good− Hipparcos observations of stars in nearby open clusters offer, for agreement is found for several clusters (Hyades,− Praesepe, α the first time, the possibility of determining accurate distances Persei), i.e. the distances determined for the adopted metallic- to these clusters without any assumption about their chemical ity correspond to those obtained from Hipparcos. Problems are composition or about stellar structure. The new distance mod- found for the Pleiades (and Coma Ber cluster which only has ulus of the Hyades, 3.33 0.01, derived by Perryman et al. colours). PSSKH attributed these discrepancies to 1 mas ± B V (1998) is a first step in the determination of the distance scale systematic− errors in the Hipparcos Catalogue. in the universe. The high precision obtained represents an im- In fact, a more general view of the situation should be ob- portant improvement with respect to the results of decades of tained from the analysis of additional nearby open clusters. For attempts to fix the zero point of the distance scale. example, NGC 2516 which occupies the lowest position in the The position of the Zero Age Main Sequence (ZAMS) is HR diagram with respect to Praesepe (even below that of the sensitive to the exact chemical composition of the clusters and Pleiades) has a metallicity [F e/H]= 0.32 (Jeffries et al. a difference of [Fe/H] = 0.15, corresponding to the metallicity 1997), in good agreement with that required− to adequately fit difference between the Hyades and the Sun, results in a dis- the ZAMS in the colour-magnitude diagram. placement of about 0.2 magnitude in absolute magnitude (MV ) The results and detailed discussions presented in this pa- according to several internal structure and atmosphere mod- per are in keeping with preliminary results presented at the els. As the exact chemical composition of most clusters is not Venice’97 Symposium (Robichon et al. 1997). Since this Sym- presently known with the required accuracy, the metallicity cor- posium, careful investigations of possible biases have been per- rections to the distance moduli are not known with precision. formed, but no evidence of any bias larger than few tenths of a Thanks to Hipparcos observations, it is possible to determine the milliarcsecond has been discovered. Discrepancies between the Send offprint requests to:Noel¨ Robichon parallaxes of the Pleiades and Coma Ber with the ground-based ? Based on observations made with the ESA Hipparcos astrometry values of Pinsonneault et al. still exists, and an attempt to ex- satellite plain them will be given in a following paper (Robichon et al. 472 N. Robichon et al.: Open clusters with Hipparcos. I in prep.). This second paper will analyse the cluster sequences can be derived with good accuracy. Because they are quite dif- in the colour-magnitude diagram in the light of Hipparcos data. ferent from field star parallaxes and pro-per motions, a new and It will complete the analysis of the cluster sequences in several secure selection of members in the Hipparcos Catalogue can photometric systems presented in Mermilliod (1998) which ex- be performed, which replaces the pre-launch selected sample. hibits a significant correlation between the cluster metallicities This concerns all the clusters closer than 300 pc and 8 additional and their relative positions in the (MV , (B V )0) diagram clusters closer than 500 pc. when using the Hipparcos distance moduli. − For the other clusters, situated further than 500 pc or with The outline of the paper is the following. Sect. 2 depicts the a number of Hipparcos stars smaller than 8, the mean paral- two different methods adopted for selecting cluster members laxes and proper motions are small or not accurate enough and from the Hipparcos astrometric data, depending on whether or members are harder to separate from field stars on an astromet- not they are closer than 500 pc and contain at least 8 members. ric basis. A selection based only on astrometrical criteria would With these sets of members, the mean astrometric parameters accept non member stars and could then bias the computed mean (π, µα cos δ, µδ) of 18 rich clusters closer than 500 parsecs, and parameters of the cluster. Nevertheless, even if the mean Hip- 32 more distant and/or containing between 4 and 7 members, parcos parallax is not so precise compared to distance modulus are computed and given in Sect. 3. The method used to compute derived, for example, from a MSF, it is interesting to compute these mean astrometric parameters is briefly described. It uti- their mean astrometric parameters for at least two reasons. On lizes Hipparcos intermediate data which allow to take account the one hand, mean parallaxes of dozens of clusters allow statis- of the star to star correlations. The rest of the paper reviews the tical calibration of other distance indicators. On the other hand, possibility of systematic errors in the parameters both at large the cluster mean proper motions can be very useful for galactic scale and small scale. The conclusion of this last part is that the kinematic studies. For these clusters, only stars preselected in mean astrometric parameters are statistically unbiased over the the Hipparcos Input Catalogue were taken into account. For the sky and that their formal errors are not severely underestimated. 110 clusters farther than 300 pc and with at least 2 Hipparcos stars, the mean astrometric parameters have also been derived. 2. Selection of cluster members No attempt has been made to find new nearby clusters in the Hipparcos Catalogue. Platais et al. (1998) made a survey of new 2.1. Pre-launch selection open clusters and associations in the Hipparcos Catalogue. They The initial selection for inclusion of cluster stars in the Hippar- found some possible new clusters which need to be confirmed cos Input Catalogue (HIC) (Turon et al. 1992) is described in by further analysis at fainter stars. These new objects are then detail in (Mermilliod & Turon 1989). It was based on the condi- not included in the present paper. The same goes for OB asso- tions of membership from proper motions and radial velocities ciations which are studied in detail using Hipparcos data in a when available, and the positions in the colour-magnitude di- comprehensive paper by de Zeeuw et al. 1999. The method used agram on the single star sequence to minimize the effects of here to derive cluster mean astrometric parameters is not suited potential companions.
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